Not to crap on this project too much, but this, and most of the other small CAD projects that get posted here, always have a condescending, "Look how easy I've made this" vibe to them, and then they're always light years behind modern CAD software.

If you're doing serious CAD work, like designing a whole machine, or working with multiple vendors to get parts manufactured, then it really is worth paying for a commercial CAD system and learning how to use it. All of the commercial CAD systems today support parametric modelling where later operations are updated automatically when the earlier operations are changed by the user.

If you insist on writing code, then even entry level systems like SolidWorks have APIs to do everything from creating the part geometry, defining geometric tolerances and PMI, running simulations, doing different types of analysis, creating machine/tooling instructions (G-code, etc.), exporting to various formats, and a million other things.

And they have workbenches for manufacturing techniques other than 3D printing, like sheet metal, CNC machining, cable routing, injection molding, welding, etc. And most of them have libaries of standard and off the shelf parts, like screws, bolts, nuts, washers, cables and housings, etc. in various standardized sizes so you don't have to model those at all.

For testing, on the higher end, the CAD systems integrate with metrology hardware and they're able to actually measure the manufactured parts and compare against the model to validate that they have the correct shape and meet all of the tolerances. Not as a virtual unit test running in CI, but by actually measuring the physical part.